The present invention relates to fluorescent lighting and more particularly to electronic fluorescent lighting ballast circuitry.
Modern fluorescent lamp circuits use solid state ballasts. U.S. Pat. No. 4,686,427 issued to Burke discloses an example of a typical solid state ballast circuit which converts AC line voltage into DC and then applies pulses to the lamps at a high frequency for eliminating lamp flicker and hum.
While early fluorescent lamps were operated at 60 Hz, a higher frequency of operation is desirable because as the operation frequency increases, the efficiency of the lamps also increases. The higher operation frequency can also lead to smaller and lighter weight components in the ballast circuitry and a steadier light output.
By itself, a fluorescent lamp is inherently a high-power-factor device but its ballast exhibits a low-power-factor. Thus, since a single-lamp circuit may have a power factor on the order of 50%, power factor correction is desirable. Without power factor correction load current will be out of phase with the line voltage and therefore, to produce a certain amount of light, the circuit must draw additional current from the power line. For example, a circuit operating at 115 volts to produce 1200 watts of power would apparently require approximately 10.4 amps of current. However, with a power factor of, for example, 65%, the circuit would draw approximately 16 amps to produce the same amount of work. Thus, wiring and circuit breakers in the lighting system would have to be of larger size than if the system had a higher power factor. Present day fluorescent lamp ballast circuits typically include components for power factor correction. Such components will, for example, comprise the addition of capacitance to bring the voltage and current closer into phase. A disadvantage to the sole use of these components is that the size, cost and weight of additional circuitry can be relatively large. In addition, the power factor correction achieved may not be as large as desired.
Lamp control circuits have used lamp light output as an indicator of lamp current. This use of light output may not result in accurate lamp current control since, for example, hot cathode fluorescent lamp light output at a given current is dependent upon air temperature surrounding the lamp. For example, a hot cathode lamp with a given current flow will produce approximately 70% of the light at 30.degree. F. that the lamp would emit if the temperature were 80.degree. F. Therefore, employing light output as an indicator of current can lead to excessive lamp current. Other circuits use a measure of current flow in the primary of a lamp driver circuit as an indicator of actual lamp current. However, this method does not provide a true representation of the current flowing through the lamps. Still other ballast circuits employ additional transformer windings or even separate transformers in the lamp circuit to monitor current flow. It would be desirable to measure the actual current flow through the lamps and adjust the lamp drive current based on that measurement without the need for transformers.